(a) Field of the Invention
The present invention relates to a powertrain for an automatic transmission and a hydraulic control system for controlling the powertrain.
(b) Description of the Related Art
Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism, the gear shift mechanism being realized through a planetary gearset. The friction elements are controlled to engaged and disengaged states by a hydraulic control system, which controls pressure generated by an fluid pump, to change shift ratios of the planetary gearset.
The friction elements are selectively operated by a plurality of valves, which undergo port conversion to change the flow of hydraulic pressure, and actuators supplying hydraulic pressure to the valves. Further, a manual valve, indexed with a driver-operated shift selector to realize port conversion, is connected to a plurality of lines to supply hydraulic pressure from the fluid pump to each valve and actuator.
Solenoid valves are operated to ON and OFF states in different combinations to realize control into the various speeds and shift modes. That is, as described above, the solenoid valves are operated to ON and OFF states such that the supply of hydraulic pressure to the valves is controlled. This, in turn, controls the supply of hydraulic pressure to specific friction elements to control the same, thereby ultimately controlling the multi-stage gear shift mechanism for control into the different shift speeds and modes.
However, in the prior art hydraulic control system, an engine brake is applied frequently at unsuitable times such that drive performance is reduced. Also, fail-safe means in case the transmission malfunctions is not provided.
The present invention has been made in an effort to solve the above problems.
It is a first objective of the present invention to provide a powertrain and a hydraulic control system for controlling the same that can apply an engine brake at only the highest speed of each drive range, thereby improving shift quality.
It is a second objective of the present invention to provide a powertrain and a hydraulic control system for controlling the same that can provide a dual fail-safe mode such that in a drive D range, a fourth speed hold is realized, and in a drive D3 range, a third speed hold is realized, thereby improving the drive performance.
It is a third objective of the present invention to provide a hydraulic control system that can improve shift quality by performing a release control using a one-way clutch throughout whole shift speeds.